Arrays of light emitting devices, such as inorganic light emitting diodes, organic light emitting diodes or electroluminescent devices, etc., are becoming increasingly popular in displays. This popularity is chiefly due to the low operating voltages and power required and to the small size, as well as the ease of manufacturing. This is especially true of organic electroluminescent devices (OEDs), since manufacturing is even easier than many other light emitting devices, the brightness is even greater, and the voltage requirements are substantially less.
Also, organic electroluminescent device (OED) arrays are emerging as a potentially viable design choice for use in small products, especially small portable electronic devices, such as pagers, cellular and portable telephones, two-way radios, data banks, etc. OED arrays are capable of generating sufficient light for use in displays under a variety of ambient light conditions (from little or no ambient light to bright ambient light). Further, OEDs can be fabricated relatively cheaply and in a variety of sizes from very small (less than a tenth millimeter in diameter) to relatively large (greater than an inch) so that OED arrays can be fabricated in a variety of sizes. Also, OEDs have the added advantage that their emissive operation provides a very wide viewing angle.
Generally, OEDs include a first electrically conductive layer (or first contact), an electron transporting and emission layer, a hole transporting layer and a second electrically conductive layer (or second contact). The light can be transmitted either way but must exit through one of the conductive layers. There are many ways to modify one of the conductive layers for the emission of light therethrough but it has been found generally that the most efficient OED includes one conductive layer which is transparent to the light being emitted. Also, one of the most widely used conductive, transparent materials is indium-tin-oxide (ITO), which is generally deposited in a layer on a transparent substrate such as a glass plate.
The problem is that most of the light emitting devices have an internal capacitance which charges during operation and remains charged after activation of the device. This lingering or persisting charge produces what is commonly referred to as shadows in the display, because the charge remaining from a previous row of data will continue to produce light emissions that appear in the next row of data and, in some circuits, may effect the next row of data. Because OEDs include a layer of organic material, which is nearly a dielectric, sandwiched between layers of conducting material, the internal capacitance is relatively large. Thus the shadow problem is especially prevalent in arrays of OEDs.
Accordingly, it would be highly desirable to produce an array of light emitting diodes in which no shadow is prevalent.
It is a purpose of the present invention to provide a new and improved array of light emitting devices with active pull down shadow canceling.
It is another purpose of the present invention to provide a new and improved array of light emitting devices with active pull down shadow canceling which adds very little complexity to the array.
It is still another purpose of the present invention to provide a new and improved array of light emitting devices with active pull down shadow canceling which is relatively simple and inexpensive to manufacture.
It is a still further purpose of the present invention to provide a new and improved array of light emitting devices with active pull down shadow canceling which is easily integrated on a common substrate with little additional cost and effort.
It is also an object of the present invention to provide new and improved means for scanning an organic electroluminescent array display.
It is a further object of the present invention to provide organic electroluminescent displays which are less expensive, smaller, and easier to manufacture.
It is still yet another object of the present invention to provide electroluminescent displays with reduced shadow effects in image generation.